Atomic layer deposition (ALD) provides highly conformal material coatings with exceptional quality, atomic layer control, and uniformity. Coatings deposited by ALD are, for example, well suited for protecting many products from corrosion and harsh ambient conditions. Effective corrosion protective ALD coatings may only be about 200 to about 1,000 nanometers (nm) thick, making them thin enough not to impact the dimensions or the bulk properties of most of the parts and products on which they are deposited. Moreover, ALD coatings typically display excellent conformality and hermetic sealing properties. As a result, potential applications for ALD coatings are wide ranging. They include microelectronic packaging, medical devices, microelectromechanical systems, carbon nanotube assemblies, flat panel displays, high-end consumer and aerospace parts, printed circuit boards, tools, solar panels, and a myriad of other applications.
Fundamentally, repetitive ALD process cycles consist at the very minimum of two reaction sub-steps. Typically, in a first reaction sub-step, a substrate is exposed to a first precursor gas MLx having a metal element M (e.g., M=Al, W, Ta, or Si.) that is bonded to an atomic or molecular ligand L. The substrate surface is typically prepared to include hydrogen-containing ligands AH (e.g., A=O, N, or S). These hydrogen-containing ligands react with the first precursor gas to deposit a layer of metal by the reaction:substrate−AH+MLx→substrate−AMLx-1+HL  (1)where the hydrogen containing molecule HL is a reaction by-product. During the reaction, the AH surface ligands are consumed, and the surface becomes covered with L ligands from the first precursor gas, which cannot react further with that gas. As a result, the reaction self-terminates when substantially all the AH ligands on the surface are replaced with AMLx-1 species. This reaction sub-step is typically followed by an inert-gas (e.g., N2 or Ar) sweep sub-step that acts to sweep substantially all of the remaining first precursor gas from the process space in preparation for the introduction of a second precursor gas.
The second precursor gas is used to restore the surface reactivity of the substrate towards the first precursor gas. This is done, for example, by removing the L ligands on the substrate and re-depositing AH ligands. In this case, the second precursor gas typically consists of AHy (e.g., AHy=H2O, NH3, or H2S). The reaction:substrate−ML+AHy→substrate−M−AH+HL  (2)converts the surface of the substrate back to being AH-covered (note that this reaction as stated is not balanced for simplicity). The desired additional element A is incorporated into the film and the undesired ligands L are substantially eliminated as volatile by-product. Once again, the reaction consumes the reactive sites (this time, the L-terminated sites) and self-terminates when those sites are entirely depleted. The remaining second precursor gas is then removed from the process space by another sweep sub-step.
The sub-steps consisting of reacting the substrate with the first precursor gas until saturation and then restoring the substrate to a reactive condition with the second precursor gas form the key elements in an ALD process cycle. These sub-steps imply that films can be layered down in equal, metered cycles that are all identical in chemical kinetics, deposition per cycle, composition, and thickness. Moreover, self-saturating surface reactions make ALD insensitive to precursor transport non-uniformities (i.e., spatial non-uniformity in the rate that the precursor gases impinge on the substrate) that often plague other deposition techniques like chemical vapor deposition and physical vapor deposition. Transport non-uniformities may result from equipment deficiencies or may be driven by substrate topography. Nonetheless, in the case of self-saturating ALD reactions, if each of the reaction sub-steps is allowed to self-saturate across the entire substrate surface, transport non-uniformities become irrelevant to film growth rate.
As described generally above, an ALD process cycle requires two reaction sub-steps and their associated sweep sub-steps. If each reaction sub-step is further particularized into an injection sub-step, wherein the respective precursor gas is injected into the reaction space, and a reaction sub-step, then a single process cycle actually consists of six sub-steps in total:                1. MLx injection        2. MLx reaction        3. MLx sweep        4. AHy injection        5. AHy reaction        6. AHy sweep.The highest productivity is achieved when each of these sub-steps completes as quickly as possible. In fact, because it frequently requires about 2,000 ALD process cycles to complete an encapsulation process, each cycle will preferably require less than about one second. Productivity is, of course, also affected by other factors. In addition to cycle time, productivity is also affected by equipment uptime (i.e., the fraction of the time that the equipment is up and running properly), cost of consumables (e.g., precursor gases, sweep gases), cost of maintenance, power, overhead (e.g., floor space), and labor.        
Reaction rates during the reaction sub-steps tend to scale with the flux of precursor gases on the substrate, which, in turn, scale with the partial pressure of that precursor gas in the process space. Most ALD processes are performed at the low to moderate substrate temperature range of about 100-300 degrees Celsius (° C.). At these lower temperatures, reaction rates are relatively slow or only moderate in speed. As a result, substantial exposures (e.g., about 102-105 Langmuirs (L)) of precursor gas may be needed to reach saturation. In these cases, high precursor gas pressure is typically the only way to speed up the reaction sub-steps. Accordingly, reaction sub-steps are preferably executed at the highest possible pressure of undiluted precursor gas. In contrast, typically very minimal gas flow is needed during the reaction sub-steps to supplement for reactive precursor depletion. Instead, higher gas flow rates will only result in extensive precursor waste. Since many of the precursor gases used in ALD are extremely reactive, un-reacted precursor gas that is swept through the process space swiftly drives the equipment to malfunction or to fail. It is therefore preferably that reaction sub-steps are performed with the highest pressures and the lowest gas flow rates.
Effective sweep sub-steps, in contrast, preferably utilize high gas flow rates of the sweep gas to substantially remove any precursor gas from the process space before introducing the complementary precursor gas into this space. Dilution by a factor of about 100-500 during a sweep sub-step is generally considered by those who are skilled in the art to be sufficient to promote high quality ALD growth. Required sweep sub-step times scale with the sweep residence time, τs=V×Ps/Qs, where V is the volume of the process space, Ps, is the pressure of sweep gas in the process space, and Qs, is the gas flow rate of the sweep gas in the process space. Based on the 100-500 dilution criteria, effective sweep times will exceed about 4.5 τs. Based on this formula, one will recognize that, to reduce required sweep sub-step time, process space volume is preferably minimized when designing the deposition system. Moreover, sweep sub-step time may be reduced by using lower sweep gas pressures and higher sweep gas flow rates. The sweep sub-steps therefore display trends with respect to pressure and gas flow rate that are opposite to those described above for the reaction sub-steps.
Injection sub-steps drive a concurrent flow-out (“draw”) of sweep gas form the process space while it is loaded with the appropriate precursor gas. The time required for the injection sub-steps scales with injection residence time τi=VPi/Qi, where Pi is the pressure of the precursor gas in the process space, and Qi is the gas flow rate of the precursor gas in the process space. Accordingly low pressures and high gas flow rates allow the injection sub-steps to be faster. Bearing in mind, however, that precursor waste and related equipment failure, downtime, and maintenance are perhaps the most dominant cost factors, best ALD practices generally dictate that injection sub-steps not be carried out beyond 35% volume exchange (e.g., about τi) under these gas flow rate conditions. Otherwise, high concentration loading will result in excessive precursor waste during the injection sub-step. For example, to reach greater than 99% concentration of precursor gas in the process space during an injection sub-step, the required injection time of about 4.5τi will result in more than 58% precursor waste just for that injection sub-step. This restriction further emphasizes the need for high pressure during the reaction sub-steps to compensate for less than 100% concentrations of precursor gas in the process space.
Based on these trends, one can see that conventional ALD clearly suffers from a fundamental tradeoff: injection and sweep sub-steps are made faster with lower pressures and higher gas flow rates while reaction sub-steps are made faster and less wasteful of precursor gases with higher pressures and lower gas flow rates. To overcome this tradeoff, process pressure and gas flow rates are preferably modulated in a synchronized manner with the different ALD sub-steps. Nevertheless, driving higher gas flow rates in many apparatus known in the art results in higher pressures so that any advantageous effects for ALD applications are lost. For example, the residence time τ=V×P/Q does not modulate when both pressure, P, and gas flow rate, Q, are modulated in phase with each other by roughly the same factor. Moreover, pressure/gas-flow-rate modulation techniques known in the art tend to employ relatively slow mechanical devices that modulate conductance (e.g., throttle valves) or devices that modulate pumping speed (e.g., devices that change the speed at which a component of the pump moves or rotates). These devices are not practical for the sub-second execution of ALD. For efficient ALD, the time required to modulate pressure and gas flow rates should not ideally exceed 10% of the process cycle time. For example, 100 milliseconds (ms) out of a one second cycle time leaves only about 25 ms for each pressure/gas-flow-rate transition (there are four such transitions per process cycle). Excluding other drawbacks, a transition time of about 25 ms is at least 100 times faster than the speed of most mechanical and pump speed modulation methodologies.
A novel ALD apparatus and method were taught by the inventor of the present invention in U.S. Pat. No. 6,911,092, entitled “ALD Apparatus and Method,” commonly assigned herewith and incorporated by reference herein. Aspects of this invention are shown in the schematic diagram shown in FIG. 1. As indicated in the figure, a “Synchronously Modulated Flow Draw” (SMFD) ALD system 100 comprises a first precursor gas source 101, a sweep gas source 102, and a second precursor gas source 103. These sources are plumbed into a first precursor gas valve 105, a sweep gas valve 106, and a second precursor gas valve 107, respectively, which control the flow of these process gases into inlets of a process space 110. Further downstream, a process space flow restriction element (FRE) 115 is attached to an outlet of the process space and carries gas drawn out of the process space into a small-volume draw gas introduction chamber (DGIC) 116. A draw gas source 120 is connected to the DGIC through a draw gas valve 121 and a draw gas FRE 122. Any gases drawn out of the DGIC enter a DGIC FRE 130 and then an abatement space 132, which contains an abatement surface 134. The abatement space is connected to an abatement gas source 138 and an abatement gas valve 139. The system is pumped by a vacuum pump 140.
The SMFD ALD system 100 is adapted to run process cycles comprising the six sub-steps described above. During sweep sub-steps, the draw gas valve 121 is closed and no draw gas is allowed to enter the DGIC 116. This, in turn, allows sweep gases injected into the process space to achieve relatively low pressures and relatively high gas flow rates. In contrast, during injection and reactions sub-steps, the draw gas valve is opened and draw gas is injected into the DGIC, allowing precursor gases injected into the process space to rapidly achieve relatively high pressures while accommodating relatively low gas flow rates. More particularly, given the small volume of DGIC and the high flow of the draw gas, a substantial pressure gradient quickly develops over the DGIC FRE 130 when draw gas is injected into the DGIC. As a result, pressure in the DGIC quickly increases and the pressure gradient over the process space FRE 115, ΔPDraw, quickly decreases. In this manner, the gas flow rate out of the process space is modulated by effectively modulating ΔPDraw. If the DGIC has a small volume, very fast transition speeds may be obtained. For example, a DGIC having a volume of about 75 cubic centimeters (cm3) implemented within a commercially available SMFD ALD system designed to deposit materials on eight inch wafer-sized substrates is capable of less than 5 ms transition times.
For gas abatement purposes, an abatement gas from the abatement source 138 is introduced through the abatement gas valve 139 into the abatement space 132 during reaction and the initial stage of sweep sub-steps to drive an efficient reaction with any precursor gases that may have passed through the process space 110 without being reacted. The products of this abatement reaction deposit as a solid film on the abatement surface 134, thereby effectively scrubbing the leftover precursor gas waste from the exhaust effluent. Advantageously, the high gas flow rate through the DGIC 116 during reaction sub-steps effectively separates the abatement space from the process space to allow flexible abatement gas selection without affecting the actual ALD process. Abatement accomplished in this manner has been shown to extend pump life significantly over that normally seen in conventional ALD systems.
Based on this brief description as well as the details provided in U.S. Pat. No. 6,911,092, it will be clear to one skilled in the art that SMFD ALD methods and apparatus provide several advantages with respect to productivity, efficiency, and cost over other ALD methods and apparatus known in the art. For this reason, it is desirable to further develop new methods and apparatus for implementation of SMFD-like ALD which may provide even greater advantages and capabilities.